1. Field of the Invention
The invention relates to a process for modifying a hydrocarbon mixture that contains olefins. More particularly, the invention relates to using a shape-selective molecular sieve catalyst in a catalytic cracking unit having such a hydrocarbon mixture as a feed. The catalytic cracking unit is operated under conditions that result in products with a diminished concentration of sulfur-containing species and an augmented concentration of saturated species compared to the feed.
2. Background of the Invention
The need for low emissions fuels has created an increased demand for high-octane motor gasoline blend-stocks having an increased concentration of saturated species and a reduced concentration of sulfur-containing species. Moreover, a low cost supply of light olefins; particularly propylene, continues to be in demand to serve as feedstock for polyolefin, particularly polypropylene production.
In this regard, conventional fluid catalytic cracking (xe2x80x9cFCCxe2x80x9d) units may be operated to maximize olefin production to meet motor gasoline blending requirements. The operation of the unit is designed so that a suitable catalyst will act to convert a heavy gas oil to maximize either gasoline or light olefin production. Increasing production of the desired product may be achieved, for example, by using an optimal catalyst and by optimizing reaction parameters.
In yet another conventional process, a hydrocarbonaceous feedstock is converted by contacting the feedstock with a moving bed of a zeolitic catalyst comprising a zeolite with a pore diameter of 0.3 to 0.7 nm, at a temperature above about 500xc2x0 C. and at a residence time less than about 10 seconds. Olefins are produced with relatively little saturated gaseous hydrocarbons being formed. In a related process, olefins are formed from hydrocarbonaceous feedstock in the presence of a ZSM-5 catalyst.
The conventional processes may not meet current or proposed motor gasoline concentration limits for sulfur species or for olefin having molecular weight above about C5. Some conventional processes attempt to reduce sulfur and olefin concentration by employing a hydroprocessing stage subsequent to catalytic cracking. But such hydroprocessing may result in an undesirable reduction in naphtha octane number.
Therefore, there is a need for processes for forming naphthas such as high-octane motor gasoline blend-stocks having an increased concentration of saturated species.
In accordance with the present invention there is provided a process for forming a high-octane naphtha comprising:
contacting a naphtha feed having a feed Reid Vapor Pressure (xe2x80x9cRVPxe2x80x9d) and an average feed octane number and containing paraffinic species, sulfur-containing species, and olefinic species under catalytic conversion conditions with a catalytically effective amount of a catalyst containing 10 to 50 wt. % of a molecular sieve having an average pore diameter less than about 0.7 nm at a temperature ranging from about 500xc2x0 C. to about 650xc2x0 C., a hydrocarbon partial pressure ranging from about 10 to about 40 psia, a hydrocarbon residence time ranging from about 1 to about 10 seconds, and a catalyst to feed weight ratio ranging from about 2 to about 10 in order to form the high octane naphtha.
In another embodiment, the invention is a product formed in accordance with such a process.
In a preferred embodiment, no more than about 20 wt. % of the feed paraffinic species are converted to species in the high octane naphtha having molecular weights lower than about C4; the high octane naphtha has about 60 wt. % to about 90 wt. % less olefin; and the high octane naphtha has an average product octane number ((R+M)/2) substantially the same as or greater than the feed""s average octane number, and a product RVP substantially the same as or less than the feed RVP.
In another preferred embodiment, the catalyst contains about 10 wt. % to about 80 wt. % of a crystalline zeolite having an average pore diameter less than about 0.7 nm.
In another embodiment, the invention is a method for forming a high octane, low-sulfur blended gasoline, the method comprising:
(a) contacting a naphtha feed containing sulfur-bearing species and olefin under catalytic conversion conditions with a catalytically effective amount of a catalyst, wherein the catalyst contains 10 to 50 wt. % of a molecular sieve having an average pore diameter less than about 0.7 nm, at a temperature ranging from about 500xc2x0 C. to about 650xc2x0 C., a hydrocarbon partial pressure ranging from about 10 to about 40 psia, a hydrocarbon residence time ranging from about 1 to about 10 seconds, and a catalyst to feed weight ratio ranging from about 2 to about 10 in order to form a high octane naphtha, and then
(b) combining at least a portion of the high octane naphtha with a gasoline having an initial RVP and an initial average octane number in order to form a blended gasoline having an average blend octane number substantially the same as or greater than the initial average octane number, and an average blend RVP substantially the same as or less than the initial RVP.
In yet another embodiment, the invention is a blended gasoline formed in accordance with such a process.